The present invention relates to a digital filter which can be suitably applied for data communication, and more particularly, to a roll-off filter which can realize a Nyquist filter capable of eliminating intersymbol interference caused by filtering.
Generally, it is known that a digital signal has quite a broad spectrum range for its data rate. For example, when a digital signal shown in FIG. 12(a) has a data rate of 1 Mbps, then its spectrum is spread as shown in FIG. 12(b).
On the other hand, a digital signal of images or sounds is transmitted by minutely dividing an available frequency bandwidth in accordance with its use or purposes. In order to send a large volume of information within limited frequency bandwidths, a data transferring rate may be improved, or the bandwidth of a signal having a certain data transferring rate may be narrowed, so that some portions of data are multiplexed by means of frequency dividing. In particular, in the field of a wireless communication, a radio wave source is effectively utilized by narrowing the bandwidth by suppressing an unwanted sideband of a base band signal.
However, when the bandwidth of a spectrum of the digital signal is narrowed, there occurs intersymbol interference, which possibly causes a bit error. In order to solve this problem, a Nyquist filter is extensively used as a filter which does not cause intersymbol interference even when the bandwidth is narrowed.
Characteristics of a roll-off filter R(f) which are given by Nyquist and realize a intersymbol-interference-free Nyquist filter are expressed as Equation below and illustrated in FIG. 13:                               R          ⁡                      (            f            )                          =                  {                                                    1                                                              xe2x80x83                                                            …                                                              0                  ≤                                      "LeftBracketingBar"                    ft                    "RightBracketingBar"                                    ≦                                                            1                      -                      α                                        2                                                                                                                        1                  2                                                                              {                                      1                    -                                          sin                      [                                                                        π                                                      2                            ⁢                            α                                                                          ⁢                                                  (                                                                                    2                              ⁢                                                              xe2x80x83                                                            ⁢                              ft                                                        -                            1                                                    )                                                                    ]                                                        }                                                            …                                                                                                        1                      -                      α                                        2                                    ≦                                      "LeftBracketingBar"                                          f                      ⁢                      t                                        "RightBracketingBar"                                    ≦                                                            1                      +                      α                                        2                                                                                                      0                                                              xe2x80x83                                                            …                                                                                                        1                      +                      α                                        2                                    ≦                                      "LeftBracketingBar"                                          f                      ⁢                      t                                        "RightBracketingBar"                                                                                                          (        1        )            
where T is a sign interval, and xcex1 is a roll-off ratio defined as 0xe2x89xa6xcex1xe2x89xa61. The roll-off filter R(f) has been well known as a filter functioning as the Nyquist filter, and in the explanation below, the Nyquist filter is referred to as the roll-off filter.
In FIG. 13, a capital letter W denotes a transition period. The transition period W is 0 when the filter characteristics are ideal xcex1=0, and the larger the roll-off ratio xcex1, the longer the transition period W. FIG. 13 shows the transition period when the roll-off ratio xcex1 is 0.5.
As shown in FIG. 13, by completing the transition from one sign to the other within a unit sign interval 1T, even there is an interference wave indicated by a broken line in FIG. 14(b), data indicated by a solid line, that is, sign data shown in FIG. 14(a), can be reproduced from data read out at predetermined reading points indicated by circles in FIG. 14(b). Consequently, filter characteristics necessary to establish a intersymbol-interference-free transmission path can be achieved.
Also, an impulse response r(t) of the roll-off filter R(f) is expressed as:                               r          ⁡                      (            t            )                          =                                            sin              ⁡                              (                                  π                  ⁢                                      xe2x80x83                                    ⁢                                      t                    /                    T                                                  )                                                    π              ⁢                              xe2x80x83                            ⁢                              t                /                T                                              ·                                                    cos                ⁡                                  (                                      α                    ⁢                                          xe2x80x83                                        ⁢                                          t                      /                      T                                                        )                                                            1                -                                                      (                                          2                      ⁢                      α                      ⁢                                              xe2x80x83                                            ⁢                                              t                        /                        T                                                              )                                    2                                                      .                                              (        2        )            
If the roll-off filter R(f) is supplied with a random impulse train xcex4n (n= . . . , xe2x88x921, 0, 1, . . . ) having positive and negative polarities, then a resulting output is expressed as:                                           r            1                    ⁡                      (            t            )                          =                              ∑                          n              =                              -                ∞                                      ∞                    ⁢                      δ            ⁢                          xe2x80x83                        ⁢                                          nr                ⁡                                  (                                      t                    -                    nT                                    )                                            .                                                          (        3        )            
If analog elements, such as L, C, and R, are used, a highly sophisticated design using a computer is required to achieve these filter characteristics. However, these characteristics can be achieved relatively easy if an FIR (Finite Impulse Response) filter using delay circuits each equipped with a tap or a non-cyclic filter known as a digital filter is used.
FIG. 15 is a block diagram depicting an electrical arrangement of a typical FIR type conventional digital filter 1. As shown in the drawing, the typical FIR type digital filter 1 comprises delay devices d1, d2, . . . , dm cascaded in multiple stages, multipliers g0, g1, . . . , gm, and an adder circuit 2. Input data x(n) are delayed sequentially by the delay devices d1 through dm. Here, the input and output of each of the delay devices d1 through dm are used as taps, and the data at each tap are multiplied by coefficients h0 through hm by the multipliers g0 through gm, respectively. All the multiplication results are added up by the adder circuit 2, whereby output data y(n) are obtained. For ease of explanation, marks representing multi-bit data are appended only to the input data x(n) and output data y(n), but it should be appreciated that the data processed in the digital filter 1 are also the multi-bit data.
With the above-arranged digital filter 1, the greater the number m of the taps, the more satisfactory the filter characteristics. However, if the number m of the taps is increased, the number of the delay devices d1 through dm and the number of elements forming the adder circuit 2 are increased as well. In addition, since the multipliers g0 through gm occupy a large mounting space, the entire circuit size is undesirably increased with an increasing number of the multipliers g0 through gm.
On the other hand, a wireless communication, particularly a spectrum diffusion communication, has been receiving a widespread attention in recent years because it can offer advantages as to confidentiality, a volume of transmission data, transmission power, etc. However, the spectrum diffusion communication requires various kinds of digital signal processing including modulation processing to append a diffusion signal to a transmission signal and decoding processing to remove the diffusion signal from a received signal.
Thus, with respect to the digital filters employed for a personal computer designed to transmit information by means of the spectrum diffusion communication through a wireless LAN or a communication unit incorporated in a portable device, there has been an increasing need to reduce the number of components and downsize the circuit to save the mounting space, costs, power consumption, etc. Even when circuit elements are replaced with integrated circuits to reduce the entire circuit size, it is advantageous to use a least necessary number of circuits from the view points of shortening a developing period and saving developing costs.
It is therefore an object of the present invention to provide a digital filter of a simple arrangement which can reduce the number of components and save costs and power consumption while shortening the developing period.
In order to fulfill the above and other objects, a digital filter of the present invention is furnished with:
(a) delay circuits, cascaded in multiple stages and each having a tap, for sequentially delaying actual input data;
(b) a plurality of first adding circuits for adding up outputs from the taps supplied with a same multiplying coefficient among multiplying coefficients used to multiply an output from each tap;
(c) a plurality of multiplying circuits for multiplying an output from each of the first adding circuits with their respective multiplying coefficients;
(d) a second adding circuit for adding up multiplication results from each of the multiplying circuits and outputting an addition result as interpolation data; and
(e) an input data converting section for receiving a transfer clock having a frequency of Nxc2x7fs, and converting the actual input data in such a manner that the actual input data are outputted to the delay circuits for a 1/N period of a sign interval T of the actual input data and xe2x80x9c0xe2x80x9d is outputted to the delay circuits for a remaining period,
where N is a multiple of oversampling conducted by computing the interpolation data from the actual input data obtained by sampling input data for each sign interval T, and subsequently inserting the interpolation data into the actual input data, and fs represents a sampling frequency of the input data.
According to the above arrangement, the input data are inputted for a 1/N period of a sign interval T of the input data and xe2x80x9c0xe2x80x9d is inputted as the input data for the rest of the interval T. Thus, even when the input data shape a rectangle waveform pulse, a resulting state is identical with the one when an impulse train is inputted. Hence, since the impulse response characteristics of the roll-off filter can be utilized, a roll-off filter with optimal characteristics can be assembled with an arrangement such that the outputs from each pair of taps supplied with the same coefficient due to symmetry of the impulse response characteristics are added to each other, and each addition result is multiplied with their respective coefficients.
Consequently, since the number of elements, particularly the number of elements forming the multipliers can be reduced, the number of components can be reduced, and the costs and power consumption can be saved while the developing period can be shortened. Accordingly, the digital filter arranged in the above manner can be suitably used for a data communication device adopting the spectrum diffusion communication.